Communications devices, such as mobile telephones or personal computers, allow a subscriber to attach to a communication network and communicate with other devices.
Furthermore, a growth area is that of machine to machine (M2M) communication, in which communications are sent between different devices without human intervention. Examples of the use of M2M communication include sensor networks (for example, networks for monitoring weather conditions,), surveillance equipment (for example alarm systems, video monitoring, and so on), vehicle fleet management, vending machines, monitoring manufacturing and so on.
It is predicted that in the long term future, there will be billions of M2M devices, and the number of M2M devices will far exceed the number of devices used for communication between humans (such as mobile telephones, personal computers and so on).
When a device wishes to attach to an existing 3GPP mobile access network, it must register with the network and be authenticated. Registration and authentication are handled using information contained in a Subscriber Identity Module (SIM) or Universal Subscriber Identity Module (USIM) at the device. Each device is uniquely identified by an International Mobile Subscriber Identity (IMSI) that is stored at the SIM/USIM. The IMSI is a number range managed by ITU-T, where the number is split into three sections as follows:    1. Mobile Country Code (MCC) that uniquely identifies the country (three digits).    2. Mobile Network Code (MNC) that identifies the home Public Land Mobile Network (PLMN) of the mobile subscriber (two or three digits).    3. Mobile Subscriber Identification Number (MSIN) that identifies the mobile subscriber within a PLMN (nine to ten digits).
By way of example, if the MCC is 429, the MNC is 01, and the MSIN is 1234567890, then the IMSI is 429011234567890.
If the prediction of billions of devices holds, some countries will start to have a shortage of IMSI numbers, both for “human” subscribers and for M2M devices.
One approach to this problem is to associate a user subscription with a combination of both the IMSI and an International Mobile Equipment Identity (IMEI). The IMEI identifies the device used, and so this solution limits subscriptions to a particular device. For M2M devices, this is unlikely to be a problem.
There are various other techniques that may be used to effectively increase the number of IMSIs available. However, whichever technique is used, a problem may occur if a device moves out of its home network and attempts to attach to a visited network. If the visited network does not support a different format of subscription identifier (such as a combination of an IMSI and an IMEI, as described above) then the device will not be able to attach to the visited network.